Process mechanics in ball burnishing biomedical magnesium–calcium alloy

Magnesium–calcium (MgCa) alloys have received considerable attention recently in making biodegradable bone implants. However, the fast corrosion rate of MgCa materials imposes a challenging issue for clinical applications. Ball burnishing has emerged as a promising manufacturing alternative to tailor surface integrity of implants with the ultimate goal to increase their corrosion resistance. Ball burnishing mechanics is essential to understand the process. The process mechanics is further complicated by the normal force reduction due to unavoidable hydraulic pressure loss at the tip of the burnishing tool, and the penetration depth reduction due to elastic recovery of the workpiece material. In this study, the measured normal force shows a maximum 23?% reduction compared to theoretical value. The normal force drop is not uniform but increases with increasing applied pressure. A 2D axisymmetric and semi-infinite finite element analysis (FEA) model has been developed and validated to predict the amount of elastic recovery after burnishing. The dynamic mechanical behavior of the material is simulated using the internal state variable plasticity model and implemented in the FEA simulation using a user material subroutine. The simulated dent geometry agrees with the measured ones in terms of burnishing profile and depth. Simulation results suggest an 8?% elastic recovery on average. Acoustic emission signals are also recorded and the likely correlation with predicted residual stress, plastic strain, and temperature distributions are studied to achieve an in-process monitoring.